1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device suitable for displaying a motion picture.
2. Description of the Background Art
Liquid crystal display devices have been used in personal computers, word processors, amusement devices, television sets, etc. Studies and researches have been made in order to further improve the response characteristic of liquid crystal display devices so as to realize a high-quality motion picture display.
Japanese Laid-Open Patent Publication No. 4-288589 discloses a liquid crystal display device in which the rise response speed and the fall response speed are increased by supplying input image signals, in which high frequency components have been emphasized in advance, to the liquid crystal display section, in order to increase the response speed in a gray level display so as to reduce the after-image. Note that the “response speed” of a liquid crystal display device (liquid crystal panel) corresponds to the inverse number of an amount of time (response time) that is required for bringing the liquid crystal layer into an orientation that corresponds to the applied voltage. A configuration of a driving circuit of the liquid crystal display device will be described with reference to FIG. 14.
The driving circuit of the liquid crystal display device includes an image memory circuit 61 for storing at least one field image of an input image signal S(t), and a time base filter circuit 63 for detecting a level change in the time base direction for each picture element from the image signal stored in the image memory circuit 61 and the input image signal S(t) and for providing high-frequency-emphasizing filtering in the time base direction. The input image signal S(t) is one of R, G and B signals obtained by dividing a video signal. Since the same process is performed for the R, G and B signals, only one channel is illustrated herein.
The input image signal S(t) is stored in the image memory circuit 61 for storing at least one field of image signal. A subtractor 62 obtains the difference between a picture element signal of the input image signal S(t) and that from the image memory circuit 61, and thus serves as a level change detection circuit for detecting a change in the signal level over one field. The difference signal Sd(t) in the time base direction obtained by the subtractor 62 is input to the time base filter circuit 63 together with the input image signal S(t).
The time base filter circuit 63 includes a weighting circuit 66 for multiplying the difference signal Sd(t) with a weighting coefficient α according to the response speed, and an adder 67 for adding the input image signal S(t) to the weighted difference signal. The time base filter circuit 63 is an adaptive filter circuit capable of changing its filter characteristics according to the output from the level change detection circuit and the input level for each picture element of the input image signal. The high frequency components of the input image signal S(t) are emphasized in the time base direction by the time base filter circuit 63.
The obtained signal in which the high frequency components have been emphasized is converted to an alternating current signal by a polarity inversion circuit 64 and is supplied to a liquid crystal display section 65. The liquid crystal display section 65 is an active matrix liquid crystal display section having a display electrode (referred to also as “picture element electrode”) at each intersection between a plurality of data signal lines and a plurality of scanning signal lines extending perpendicular to the data signal lines.
FIG. 15 is a signal waveform diagram illustrating how the response characteristic is improved by the driving circuit. It is assumed that the input image signal S(t) changes at a cycle of one field for ease of understanding, and FIG. 15 shows a case where the signal level changes rapidly over two fields. In this case, the change in the input image signal S(t) in the time base direction is represented by the difference signal Sd(t), which takes a positive value for one field when the input image signal S(t) changes in the positive direction and takes a negative value for one field when the input image signal S(t) changes in the negative direction, as illustrated in FIG. 15.
Basically, the high frequency components can be emphasized by adding the difference signal Sd(t) to the input image signal S(t). In practice, since the relationship between the degree of change in the input image signal S(t) and that in the transmittance is dependent on the response speed of the liquid crystal layer, the weighting coefficient α is determined so that a correction can be made within a range such that an overshoot does not occur. As a result, a high frequency corrected signal Sc(t), in which the high frequency components have been emphasized, as illustrated in FIG. 15, is input to the liquid crystal display section. Therefore, it is possible to obtain an optical response characteristic I(t) (solid line), which is improved over that obtained by a conventional method (broken line).
Moreover, Japanese Laid-Open Patent Publication No. 2000-231091 discloses that in a case where a pixel is to be brought to a greater transmittance in a liquid crystal display device in which the liquid crystal molecules are aligned substantially vertically in the absence of an applied voltage, it is possible to reduce the response time for a transition from a black display to a low-brightness intermediate gray level display by applying a voltage that is greater than a target driving voltage to the pixel electrode.
There is a demand for a liquid crystal display device in which liquid crystal molecules respond quickly to an applied voltage. It is known in the art to employ a double-speed driving method or a backlight impulse driving method in order to obtain a high quality motion picture display with no blurredness. In order to effectively perform these driving methods, it is of course required for the liquid crystal layer to respond within one field, and it may also be required to realize a higher response speed as those achieved by the liquid crystal display devices described in the above publications.
The present invention has been made in view of the above, and has an object to provide a liquid crystal display device in which the rising response characteristic is further improved.
The term “rise” as used herein refers to a change in the display state (or the orientation of the liquid crystal layer) in response to an “increase” in the voltage applied across the liquid crystal layer. Thus, a “rise” is a change in response to an increase in the applied voltage, and corresponds to an “increase in brightness” in a normally black mode (hereinafter referred to as “NB mode”) and to a “decrease in brightness” in a normally white mode (hereinafter referred to as “NW mode”). In other words, a “rise” is associated with the orientation of the liquid crystal layer (liquid crystal molecules) being brought under tension.